Many metals are susceptible to corrosion. In this regard, atmospheric corrosion is of particular concern. Such corrosion may affect the performance and/or appearance of the metals affected, and the products produced therefrom. In addition, when polymer coatings such as paints, adhesives or sealants are applied to the metal, corrosion of the underlying metal may cause a loss of adhesion between the polymer coating and the base metal. A loss of adhesion between the polymer coating and the base metal may similarly lead to corrosion of the metal. Aluminum and aluminum alloys frequently require corrosion protection and improvements in adhesion between the base aluminum (or aluminum alloys) and subsequent polymer coatings. Aluminum alloys, in particular, can be susceptible to corrosion since the alloying elements used to improve the metal's mechanical properties may decrease corrosion resistance.
Traditionally, precipitation grade hardened high strength Al alloys containing heterogeneous microstructures formed from intermetallic compounds of Cu, Mg, Fe and Mn, are used with protective coating systems containing CrIV, chromates, and dichromates that are especially effective at inhibiting Al alloy corrosion. The corrosion resistant aircraft coating systems for aluminum substrates typically consist of a conversion coating layer, a primer layer, and a topcoat. In practice, chromate conversion coatings (“CCC”) can either be deposited on the Al surface anodically by an applied anodic current, or developed chemically by the reaction of a tri- and hexa-valent chromium salt solution (i.e., Alodine™) with the Al metal. In spite of the widespread success and use of CCC systems in protecting Al aircraft structures, as well as Cr-pigmentation, the use of chromates is being curtailed as they have been found to be carcinogenic, to be expensive to handle, and to the source of one of the highest airplane maintenance costs.
Generally, corrosion processes describe the oxidation of a metal at its surface which acts to weaken and/or disfigure it. Most metals are active enough to be converted to their oxides, and it is generally accepted that corrosion occurs by an electrochemical action involving the creation of small galvanic cells on the surface of the metal. It has been observed that the bulk of structural corrosion damage to aging aircraft emanates from components involved in the joining process to the airframe itself, such as rivets, fasteners, lap splices, joints, and spot welding. All of these joining methods are associated with metallurgical and environmentally induced factors that affect the alloying elements in the metal, and, once changed, the exterior and interior surfaces of the aircraft become more susceptible to corrosion. For example, in the one fleet of aircraft, crevice corrosion occurring in the spot welded lap joint/doubler and environmentally induced corrosion around steel fasteners on the upper wing skins have been observed and addressed as major corrosion issues.
Categorically, there are three broad factors associated with corrosion processes in aluminum alloys: 1) metallurgical; 2) mechanical, and 3) environmental. Metallurgically induced factors include heat treatment, chemical composition of the alloying elements, material discontinuities, for example the presence of voids, precipitates, grain boundaries/orientation, and/or copper concentration in second-phase (S-phase). Mechanical factors include cycle-dependent fatigue and fatigue crack initiation. Further, environmental factors contributing to corrosion include temperature, moisture content, pH, electrolyte, type of salt present, and frequency and duration of exposure.
The most widely accepted factors contributing to corrosion processes in military aluminum aircraft structures, are direct chemical attack (e.g., aggressive phosphate ester hydraulic fluid leaks), galvanic corrosion (e.g., when metals of different electrochemical potential are in contact in a corrosive medium), crevice corrosion (e.g., when a corrosive liquid gains access to crevices in or between components), pitting corrosion (e.g., a localized attack that leads to the formation of deep and narrow cavities), and stress corrosion (e.g., when tensile stress or critical environment conditions cause dealloying to occur at grain boundaries which results in the formation of anodic precipitate areas). Overall, among all of these corrosion types, material thinning by pitting at particle sites is the most basic corrosion mechanism affecting Al 2024 T-3 fuselage skin material.
Generally, Al 2024 T-3 is used for the exterior fuselage, wing skins, and flight control surfaces, where pits are observed to form in exposed grain structure when subjected to environmental conditions that favor pitting. Pitting corrosion in Al 2024 T-3 occurs when cathodic particles (al, Cu, Fe and Mn) dissolve in the alloy matrix while anodic particles (Al and Mg) also dissolve, leading to intergranular corrosion. It has been approximated that there are roughly three times more anodic particles than cathodic particles in Al 2024 T-3, and, therefore, it is prone to intergranular corrosion induced by pitting. The failure to address airframe corrosion damage due to shallow pitting, or damage related to fatigue and cracking can lead to catastrophic consequences, such as, incidents in the mid 1950s when two Comet airplanes failed in high altitude flight and the Aloha Airlines incident of 1988.
As mentioned above, prior art techniques for improving corrosion resistance of metals widely employ the use of chromate conversion coatings to passivate the surface. Such chromate treatments are undesirable, however, because the chromium used is highly toxic, carcinogenic, and environmentally undesirable. Phosphate conversion coatings are also used, but generally provide substantially less corrosion protection unless used in conjunction with a chromate.
Recently, various techniques for eliminating the use of chromates in corrosion inhibition and adhesion promotion treatments have been proposed. However, many of these proposed techniques have been proven to be ineffective, or to require time consuming, energy inefficient, multi-step processes. Thus, there remains a need for a simple, low cost, effective technique for inhibiting corrosion of metals, particularly for aluminum and aluminum alloys. The present invention, at least in part, is directed to meeting this need.